The truck shows up and the mud is too stiff to pump.

Do not allow arbitrary water addition on site. It alters the water-cement ratio (w/cm), voiding the mix design, reducing strength, and increasing shrinkage/cracking risk.

The Rules (ACI 301 / ASTM C94):

1. Pre-Placement Only: Water may be added once upon arrival before any concrete is discharged, only if: The slump is below the specified minimum. The total water added does not exceed the maximum allowable water established by the mix design (held by the producer). The truck drum rotates at mixing speed (12–15 rpm) for minimum 30 revolutions* (or 1 min/yd³) after water addition.
2. Documentation: The driver must record gallons added on the delivery ticket. You should note time, slump before/after, and gallons on your pour card.
3. Hard "No" If: Concrete has already been placed (even one chute). Adding water would exceed the design w/cm limit. Slump is already at or above the specified max. You are using high-range water reducers (superplasticizers)—adding water negates the admixture and causes segregation.

Better Solutions:

• Order the correct slump (e.g., 5"–7" for pump mixes).
• Use Mid-Range/High-Range Water Reducers dosed at the plant or via onboard system.
• Reject the load if slump is out of spec and cannot be legally corrected. Document rejection on ticket.

Bottom line: If the driver doesn't have the mix design's "water remaining" allowance written on the ticket, he cannot legally add water. Reject the load or call the plant for a chemical adjustment.

Adding water to concrete on-site to improve workability is generally not permitted under standard construction practices and codes unless explicitly allowed by the mix design and approved by the engineer of record.

Key Rules & Guidelines:

1. ASTM C94 / ACI 301 – The standard specification for ready-mixed concrete prohibits adding water beyond the design mix unless: - The water is added only to restore slump lost due to delay, not to compensate for stiff mix. - The total water added does not exceed the maximum allowable water specified in the mix design. - The addition is documented (time, amount, slump before/after) and approved by the purchaser’s representative (e.g., inspector or engineer). - The concrete is used within 90 minutes of initial mixing (or as specified).

1. Slump Limits – Adding water increases slump but reduces compressive strength, increases permeability, and can cause segregation or bleeding. For every 1 gallon of water added per cubic yard: - Compressive strength drops ~200–300 psi. - Slump increases ~1 inch.

1. When It’s Not Allowed: - If the mix is stiff due to early setting, contamination, or incorrect batching, adding water masks a quality issue and is unacceptable. - If the truck has been on-site >90 minutes (or as per local spec), retempering is usually prohibited. - Many public works projects (DOT, federal jobs) ban water addition entirely without prior written approval.

1. Best Practice: - Do not add water without direction from the supervisor or engineer. - If the mix is too stiff, reject the load and request a new batch with proper slump. - Use admixtures (e.g., mid-range water reducers) if slump loss is anticipated — never just water.

Bottom Line: Adding water is a last resort, strictly regulated, and often prohibited. Unauthorized addition risks structural integrity and liability. When in doubt, reject the load and consult the engineer. Never compromise strength for convenience. (Ref: ASTM C94, ACI 301-20, ACI 211.1)